We probably over-simplified that blog post. It's being updated, but here's where it came from, for those interested in the minutia: key size and compatibility are uncontroversial, the perf story is harder. Basically: ECDSA saves CPU/time on the server side, but not the client ..
Here's "openssl speed" for a simple example, ECDSA Vs RSA for verify. In wall-clock time: we've measured about an 80 microsecond hit. Keep in mind that ACM Private CA is for client certificates. 
Our EC2 network RTTs are in tens of micros too, so it can show up. Moral of the story: ECDSA can slow things down end-to-end. But probably too nuanced a take for a launch blog post.
